1. Field of the Invention
This invention relates generally to improvements in trans-horizon navigation systems, and, more particularly, but not by way of limitation, to a trans-horizon navigation system wherein a predetermined sequence of control pulses is phase-encoded onto an RF carrier for transmission to a receiver having a correlator responsive to the sequence of control pulses.
2. Description of the Prior Art
The basic teachings for long range position determination systems of the type disclosed herein are contained in U.S. Pat. No. 3,553,691, issued to J. W. Lassiter, the inventor of the present invention. In this Patent, it was emphasized that precise tuning of transmitting and receiving components is necessary in trans-horizon navigation systems in order to compensate for the high levels of path loss inherent in over the horizon signal transmission. It was also noted in this Patent that the greater path loss experienced at higher transmission frequencies requires higher system gain in order to provide an acceptable signal to noise ratio.
In U.S. Pat. No. 3,875,571, issued to Davis, Jr. et al., a long range marine navigation system of the pulsed transmission type is disclosed. In this Patent, the transmitter RF carrier is frequency modulated via a non-linear frequency modulation signal resulting from the stretching of a short, high frequency pulse by a surface wave device of the chirped type. At the receiver end, the frequency modulation signal is extracted in a conventional manner and compressed into a short, high frequency pulse approximating the original pulse by a surface wave device of the chirped type which is matched in characteristics to the surface wave device contained in the transmitter.
In U.S. Pat. No. 3,889,258, issued to Holmes et al., the pulse transmission system of U.S. Pat. No. 3,875,571 is employed in a navigation ranging system wherein individual base stations are responsive to unique pulse position codes which are frequency modulated onto a selected RF carrier via surface wave devices of the chirped type in a mobile station. In this Patent, the interaction between the mobile station and the several base stations is accomplished via a complex series of control and ranging pulse sequences.
In general, the techniques taught in U.S. Pat. No. 3,553,691 provide a basic foundation for all modern trans-horizon navigation systems. Further, the refinements in matched filter pulse transmission techniques taught in U.S. Pat. No. 3,875,571 have enabled improvements in noise rejection at comparable system power and loop gain levels. In addition, the pulse position discrimination technique taught in U.S. Pat. No. 3,889,258 enabled complete trans-horizon navigation systems to operate on single transmission channels rather than being spread over several channels. However, all such systems have been found to be generally unsatisfactory in operation for various mechanical and theoretical reasons, particularly when the distances between the mobile station and the base stations is in excess of 350 miles. Further, systems constructed in accordance with these patents have been determined to be incapable of satisfactory operation within the internationally recognized navigation band from 2900 to 3100 MHz.